Fix Maniphest Excel export for tasks with descriptions beginning with "="

Summary: Fixes T2369. If you create a task with a description like `= Header =`, the Excel writer interprets it as a formula. Explicitly set the cell types to strings to avoid this.

Test Plan: Exported a task with the description `=1,`; no exception after this patch.

Reviewers: btrahan, chad, vrana

Reviewed By: chad

CC: aran

Maniphest Tasks: T2369

Differential Revision: https://secure.phabricator.com/D4567